1. Field of the Invention
The present invention relates to a screw head of an injection molding machine and, more particularly, is directed to an increase in torsional and fatigue strength of the screw head.
2. Description of the Related Art
In an injection molding machine, as shown in FIG. 2, a resin material, supplied from a hopper 11, is plasticized by rotation of a screw 1. The plasticized resin material is injected into the cavity of a mold 12 to thereby injection-mold a desired article.
The screw 1 has a screw head 2 at its distal end portion. A ring 17, for preventing the reverse flow of the molten resin, is slidably arranged on a trunk portion 3 of the screw head 2. The injection molding machine shown in FIG. 2 has, in addition to the above structure, a barrel 13, an injecting cylinder 14, an injecting piston 15, and a screw rotating gear 16.
As shown in FIG. 1, the screw head 2 has a screw portion 5 and is mounted on the screw 1 by turning the screw portion 5 into the distal end portion of the screw 1.
The injection pressure of the resin repeatedly acts on the roots of the threads of the screw portion 5 of the screw head 2. The injection pressure can undesirably cause fatigue failure in the roots of the threads.
For this reason, an undercut portion 6 is provided between the trunk portion 3 and the screw portion 5 of the screw head 2, which reduces stress concentration at the roots of the threads.
According to, e.g., Jpn. UM Appln. KOKOKU Publication No. 62-45864, the diameter of the undercut portion is set in the range of 0.4 to 0.6 the diameter of the screw portion for the purpose of equalizing the fatigue strength of the screw portion and that of the undercut portion. According to "Stress Concentration" written by Masataka Nishida, the diameter of the undercut portion is set to 1/1.4 (=0.71) the diameter of the screw portion for the purpose of equalizing the stress concentration coefficient of the screw portion and that of the undercut portion.
However, in addition to the fatigue failure caused by the repeated stress generated in the screw portion by large injection pressure at the distal end portion of the screw head, the present inventors point out the following fact as the cause of the failure of the screw head of an injection molding machine. Namely, when an excessive load acts on the screw head, especially upon starting of the injection molding machine, static failure is often caused by the torsion of the undercut portion. At the starting of the injection molding machine, the resin material guided to the screw head is not sufficiently melted. Since the screw is rotated while the screw head is fixed to the barrel with the resin material, the torque of the motor acts entirely on the screw head, thereby causing this static failure phenomenon of the undercut portion. Therefore, the static failure phenomenon caused by torsion at the undercut portion should be analyzed and solved as a problem inherent to the injection molding machine. In other words, this problem is in addition to the fatigue failure occurring in the screw portion, which can be analyzed on the analogy of a bolt. Although a problem of the screw portion is posed, few experiments have been conventionally conducted by supposing the screw head portion of an injection molding machine. Most of the experiments on the fatigue of the screw are conducted as fatigue tests under tension and compression in which the screw portion is not clamped. Therefore, it is conventionally difficult to obtain an optimum shape of the screw head.
With the diameter of the undercut portion disclosed in the aforementioned prior art, the torsional strength of the undercut portion becomes lower than that of the screw portion. Thus, excessive load acting on the screw head upon starting of the injection molding machine cannot be tolerated.
Jpn. UM Appln. KOKAI Publication No. 2-101714 discloses a lock nut method in which only a screw head is rotated, even if an excessive load acts on it. However, this has a complicated mechanism and thus cannot be adopted, especially in a small-size injection molding machine.
In Jpn. Pat. Appln. KOKAI Publication No. 4-37515, in order to solve the drawback caused by the solution of Jpn. UM Appln. KOKOKU Publication No. 62-45864, holes are formed at the centers of a screw portion and a screw neck portion in the axial direction, and the outer diameter of the undercut portion is set equal to the outer diameter of the screw portion.
According to this proposal, some effect may be obtained since the torsional strength is inversely proportional to the cube of the outer diameter of the shaft. However, the torsional strength of the undercut portion becomes much lower than that of the screw portion. Therefore, the problem of the undercut portion failing when an excessive load acts on the screw head upon starting of the injection molding machine, which is inherent to the injection molding machine, is left unsolved.
In particular, in a small-size screw head, since the shaft has a small diameter, it is difficult to form a hole in it.